1. Field of the Invention
The present invention relates to an active gas laser device comprising integrated means of purifying the active gas.
2. Discussion of the Background
A distinction is made between several categories of gas lasers, depending on their sensitivity to operating conditions and as a function of the rate at which their active medium (gas) degrades.
A first category includes sealed gas lasers for which operation and operating conditions do not change the characteristics of the active gas or gases, or only change them very little.
The performances of this type of laser are very stable in time and no action on the active medium is necessary.
For example, this first category includes HeNe lasers, C0.sub.2 lasers with wave guides, and low power argon or krypton lasers.
A second category includes sealed gas lasers for which operation and operating conditions slowly change the characteristics of the active gas or gases. These lasers require very little technical action on the active medium.
For example, the second category includes argon or krypton lasers with an output power of the order of one Watt. A service operation is necessary on these lasers after about 1000 or 2000 hours of operation. Usually, the sealed head of these lasers is replaced.
Finally, there is a third category of lasers for which operation and operating conditions quickly change the active gas mixture. Therefore, the amplifying medium of these lasers must be regenerated frequently.
The third category includes high power CO.sub.2 lasers (of the order of a kilowatt) and excimer lasers.
Excimer lasers require servicing about once every week, and the performances of these lasers degrade continuously between two service operations.
For lasers in the third category, it is possible to work without recovering the gas, or to renew all or as part of the gas mixture, whichever is preferred. Some active media may also be purified and regenerated.
Therefore, the device according to the present invention is more particularly applicable to lasers in which the active medium may be regenerated, particularly by purification of gas contained in the laser chamber. Excimer lasers form part of the above described third category and will be explained in more detail below.
An excimer laser is a gas laser capable of emitting light within the ultra-violet spectral range, in a pulsed mode.
A cavity in an excimer laser contains a gas mixture that forms an amplifying medium and which produces the laser effect. This mixture comprises essentially an halogen compound (fluorine/chlorine) in molecular form, a rare gas such as krypton, xenon or argon, and a buffer gas, for example such as neon or helium.
The energy efficiency of the laser and the quality of the light beam emitted depend on a number of parameters and operating conditions, including the partial pressures of the various gases in the gas mixture and excitation of the amplifying medium (gas mixture).
Pre-ionization, for example by X-ray or ultra-violet radiation, or by a corona effect preliminary discharge, contributes to controlling the electrical pumping discharge, in other words controlling excitation of the amplifying medium.
The purity of gases in the amplifying medium is another parameter that is important for obtaining good energy efficiency and a high quality beam.
The purity of the gases in the amplifying medium contained in a chamber may be affected by chemical reactions. For example, these reactions take place between the excited halogen compounds and the walls of the chamber.
Furthermore, it is found that the laser cavity sealing elements may also form sources of pollution of the amplifying medium.
Finally, the quality of the gases in the amplifying medium may be degraded by parasite chemical combinations. New molecules formed by chemical combination of the various molecules of the mixture can absorb radiation and therefore reduce the performances of the laser.
In order to prevent a drop in the quality of the laser beam (stability in amplitude, beam uniformity, energy) and in the efficiency of the laser cavity, it is possible to either renew gases in the active medium, particularly by adding new halogen compounds, or to eliminate undesirable gases in the gas mixture.
In particular, a number of undesirable halogen molecules appearing in the gas mixture may be trapped by a cryogenic effect outside the laser. For example, these molecules include CC1.sub.4, CF.sub.4, HF.
Elimination of undesirable molecules requires precise control of the temperature of a cryogenic trap. This temperature varies as a function of the compounds to be eliminated and therefore as a function of the molecules initially present in the active medium mixture.
Table I below contains examples of pure gases that may be added to the mixture to regenerate it and the temperature of a cryostat capable of trapping undesirable molecules, for a number of active molecules.
TABLE I Active molecules XeCl XeF ArF KrF Pure gases added to the active mixture Ar * Kr * Xe * * F.sub.2 * * * HCl * Ne * * * * Cryostat temperature 130.degree. K 130.degree. K 90.degree. K 100.degree. K
FIG. 1 illustrates a particular example embodiment of a known type of gas purifier associated with a gas laser.
Reference 10 in the figure denotes a laser chamber containing an active amplifying medium in the form of a gas mixture. Reference 12 denotes a cryogenic trap purifier connected to chamber 10 in order to regenerate the gas mixture.
An outlet 14 from the chamber 10 is connected to the purifier 12 through a pipe 16. Gas taken from chamber 10 passes through the purifier 12 and, after being purified, in other words separated from its undesirable constituents, is reinjected into chamber 10 through a pipe 18. Pipe 18 connects the purifier 12 to a gas inlet 20 to chamber 10.
The purifier 12 includes a dust filter 22, a circulation pump 24 and a cryostat 26, in order starting from its inlet. These elements are connected to a tube 27. Furthermore, isolating valves 28, 30, are provided at the purifier inlet and outlet irrespectively.
The circulation pump 24 circulates the gas mixture from the chamber 10 through a heat exchanger 32 in the cryostat in which the gas is cooled. The gas is cooled by means of a heat conducting core 34, the first end of which is immersed in a bowl of liquid nitrogen 36. A second end of the core 34 is equipped with an electrical heating resistance 35 that precisely adjusts a heat balance in the core and therefore the temperature of gases passing through the heat exchanger 32.
The cryostat traps impurities in the gas mixture by condensation and partial crystallization of these impurities, in the bottom part of tube 27.
A reverse flow heat exchanger 38 is also provided between the gas outlet from purifier 12 and the gas inlet. This exchanger pre-cools the gas to be purified by retrieving at least part of the enthalpy from the purified cold gas.
Finally, an extraction valve 40 and a vacuum pump 42 are provided to empty the purifier 12.
Periodically, isolating valves 28, 30 of the purifier 12 are closed to isolate the purifier 12 from the laser chamber 10. The part of the tube between the valves is then heated to enable evaporation and extraction of accumulated residues of impurities, using the vacuum pump.
A second inlet 21 into the laser chamber 10 is connected to gas cylinders 44, 46 and 48 containing the rare gas, the gas containing halogen compounds and the buffer gas respectively. These gas cylinders are used to inject the new gas mixture into the chamber 10.
A device of this type is known and is commercially available. See, an article entitled "The GP-2000X Series Excimer Laser Gas Purifiers" by Oxford Lasers (1990), an article entitle "Cryogenic Processing of Excimer Laser Gas Mixtures" by Oxford Lasers (1991), and European Patent Publication No. EP-A-430 411).
Note that installations of the type described above are not included in lasers, but are available as auxiliary equipment.
A number of obstacles make it impossible to include cryogenic purification equipment directly in the laser device.
The first obstacle is related to vibrations emitted by vacuum and circulation pumps. These vibrations could disturb the stability of the laser beam.
Furthermore, a complete system including a laser and purification equipment would be particularly cumbersome and difficult to transport.
Finally, the global cost of a complete system of this type would be very high, particularly due to the complexity of the purification device.
Furthermore, purification devices conforming with FIG. 1 include a number of difficulties related to circulation of gases from the laser chamber. Pipes from the purifier must be equipped with mechanical and electromechanical means for controlling gas pressures and flows.
Furthermore, pumps used to circulate the gases must be specially designed to resist corrosive gases in the active mixture and must be capable of operating in a pressure range varying from a vacuum up to about 10 bars.
These constraints also contribute to the increasing cost of purifiers. Germen Patent Publication No. DE-A-3 130 588 describes another type of laser gas purifier in which the impurities are liquefied by reducing the pressure of the gas mixture. The pressure is reduced in several steps. The gas passage in the purifier during the pressure reduction steps is caused by an alternating set of pumps and valves.
Thus, difficulties similar to the difficulties mentioned above also occur with the device described in German Patent Publication No. DE-A-3 130 588.